Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers Article - Octobre 2017

Cédric Robert, Thierry Amand, Fabian Cadiz, D. Lagarde, E. Courtade, M. Manca, T. Taniguchi, K. Watanabe, B. Urbaszek, Xavier Marie

Cédric Robert, Thierry Amand, Fabian Cadiz, D. Lagarde, E. Courtade, M. Manca, T. Taniguchi, K. Watanabe, B. Urbaszek, Xavier Marie, « Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers  », Physical Review B : Condensed Matter and Materials Physics (1998-2015), octobre 2017. ISSN 1098-0121

Abstract

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the “spin-forbidden” dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting δ=0.6±0.1meV between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at Bz=0, while the upper state is partially coupled to light with z polarization (“gray” exciton). The first determination of the dark neutral exciton lifetime τD in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τD∼110±10ps for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12K.

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